The invention relates to a high pressure discharge lamp having an arc tube for operation in a generally horizontal position, said arc tube having a generally cylindrical body defining a cylinder axis, end chambers of continuously reducing cross-section, an arc discharge sustaining filling, a pair of opposing discharge electrodes arranged substantially in said end chambers, and press seals sealing each end of said arc tube in a gas-tight manner.
Lamps of this type are known from U.S. Pat. No. 4,001,623 (Howles et al) which discloses a metal halide lamp. In such lamps, the arc tube is made of quartz glass (fused silica) to withstand the high operating temperature of the arc discharge. The discharge sustaining filling is typically comprised of mercury and a starting gas, along with one or more metal halides such as sodium and scandium halides to improve the color rendering of the lamp.
When a metal halide arc tube is operated horizontally, the arc arches or bows upward due to convection currents within the discharge space. This tends to overheat the upper wall of the arc tube, which leads to a shortened lamp life. The bowed arc also causes an uneven temperature profile between the upper and lower walls of the arc tube, leading to increased condensation of the lamp fill material as compared to a similar vertically operated lamp. This adversely effects photometric parameters such as correlated color temperature (CCT), color rendering (CRI), and luminous efficacy. Thus, arc tubes intended for horizontal operation typically include design features to alleviate these problems.
For example, the above-mentioned Howles patent discloses an arc tube having a cylindrical body with vertically oriented press seals and asymmetric end chambers in which the discharge electrodes extend axially but are offset from the cylinder axis towards the lower wall in the plane of the press seal. This lowers the arc away from the upper wall to provide a more uniform temperature distribution. In a further embodiment, the tipper wall has the shape of a catenary to further improve the temperature profile. U.S Pat. No. 5,055,740 (Sulcs) discloses a similar arc tube in which the greatest length of the arc tube is at the elevation of the electrodes. U.S. Pat. No. 4,056,751 (Gungle et al) discloses an alternative design in which the arc tube is arched to match the shape of the discharge arc during lamp operation. Gungle's arched shape, however, requires extra glass forming steps to bend the arc tube body, and increases the effective diameter of the arc tube, making it unsuitable for lamps intended for small fixtures.
A disadvantage of all of the above designs is that the press seals are vertically-oriented during horizontal operation of the arc tube. It is known from U.S. Pat. No. 4,850,500 (White et al) that end chambers typically include irregularities such as corners and crevices, inadvertently formed during pressing, where they meet the press seals of the arc tube. Thus, rather than the smoothly shaped end chamber walls shown in the Howles and Sulcs patents, in practice these lamps have been found to have crevices "C" at the juncture of the press seals and the end chamber, as shown in FIG. 1. When operated horizontally, the cold spot on the arc tube is generally on the lower wall, and typically behind the electrodes. With vertically oriented press seals, it has been found that the fill constituents tend to condense and pool in the crevices, reducing the partial pressures of the constituents. The crevices are the source of a larger than desired spread in photometric parameters among a given number of lamps due to the variation in the size and location of the crevices produced during pressing.
In White et al, the corners are reduced or eliminated by an additional, secondary pressing operation normal to the major press which forms notches, or "dimples", at the juncture of the arc tube body and press seal. However, the secondary pressing operation is an additional manufacturing step, requiring additional press jaws and modified pressing equipment, which adds to lamp cost. Furthermore, White's arc tube has a straight cylindrical body with centered axially extending discharge electrodes. This construction suffers from the asymmetric temperature profile of the arc tube wall due to the arched discharge arc as discussed above.
U.S. Pat. No. 5,016,150 (Gordin et al) discloses an embodiment of an HID lamp in which the press seals are horizontally oriented and the electrodes are aligned on the cylinder axis. The lower wall of the arc tube is locally flattened to move it closer to the discharge arc (FIGS. 2A and 2B), which requires the extra steps of heating the arc tube along its lower wall and then pressing it flat. In FIG. 2A, the dashed line represents the lower wall of the arc tube prior to flattening. While reducing the temperature difference between the flattened portion of the lower wall and the upper wall, the problem of overheating of the upper wall is not addressed. Additionally, flattening of the lower wall introduces longitudinal zones "A" having a locally irregular curvature. As shown in FIG. 2b, the radius R.sub.A of these zones is larger than the nominal radius R of the unflattened portions of the arc tube. The arc tube wall in these zones is further from the discharge arc than the flattened portion and may be the undesired location for condensation and pooling of the fill constituents.